A device is already known for detecting the level of molten metal surface level within a mold in a continuous casting machine by which a local temperature variation of a mold wall is detected and on the basis of the detected signal the level of the molten metal surface can be detected. Such device has a temperature-sensitive element such as a thermocouple or a thermistor mounted on a mold wall so that the level of the molten metal surface level can be detected by detecting a variation in current, voltage or electric resistance in the temperature sensitive element caused by variation in the level of the molten metal surface level. However, since this detecting device requires lead wires extending from the temperature sensitive element on the mold wall or from the terminals thereof, not only can an accidental failure due to poor contact of the lead wires or breaking of the wires occur, but also troublesome disconnection and reconnection of the lead wires is necessary when replacement of the mold takes place, and therefore, especially in the case of tubular molds which are frequently replaced, there is the disadvantage that a lot of labor is required for changing the temperature sensitive element from one mold to the next.
Accordingly, as a solution for overcoming the above-described disadvantage, it has been proposed to use an element of thermo-sensitive magnetic substance or a combination of one element of a thermo-sensitive magnetic substance and a magnet mounted on the mold wall and a magnetic detector element near the wall, so that the level of the molten metal surface within a mold can be detected by detecting the variation in the magnetic properties of said thermo-sensitive magnetic substance caused by temperature variation of the mold wall at the mounting position of the thermo-sensitive magnetic element, and said molten metal surface level is then controlled by a control device that is operable in response to the signal from the magnetic detector, for instance, as disclosed in Japanese Pat. No. Publication No. 28,258/1973 and in prior copending U.S. application Ser. No. 741,019 filed Nov. 11, 1976. Although the above proposals are effective in that the disadvantage of the previous prior art detecting devices with respect to the connection and disconnection of the lead wires is eliminated, still there remain various problems with respect to the mounting structure of a detector element and detector element terminals, a detection error due to temperature variation, and maintenance of the detecting device, and it has been found through subsequent research that there is a need for further improvements in the device for detecting the level of the molten metal surface within a mold in a continuous casting machine.